The objective of the proposed research is to achieve quantitative reconstruction with high computational efficiency for single photon emission computed tomography (SPECT) using converging-hole collimators. Converging-hole collimators, such as fan beam and cone beam collimators with single or multiple focal points, provide better trade-off between spatial resolution and detection efficiency, hence improved imaging quality, as compared to parallel-hole collimators. They have found useful applications in three-dimensional (3D) SPECT imaging of the brain and heart in the clinics. However, to achieve quantitative reconstruction for converging beam SPECT in routine clinical use, several important problems need to be solved. First, the projection data collected from a single planar orbit cone beam geometry are not sufficient for exact reconstruction. Second, few truly 3D algorithms are currently available for reconstruction, and they require very intensive computation. The commonly used efficient algorithms make approximations which introduce artifacts and inaccuracy in the reconstructed images. Third, the design and use of multifocal converging beam collimators are in the early stage, and a truly 3D algorithm for image reconstruction remains to be developed. In addition to the special problems, physical factors, such as photon attenuation, scatter, and spatially variant detector response, degrade SPECT imaging quality. Efficient compensations for these factors are difficult for converging beam SPECT. To solve the above problems, we propose the following research aims. First, for single-orbit cone beam geometry, the dimension and location of the regions with missing data as well as the amount of missing data in a particular part of the regions will be determined to improve estimation of missing data. The results will also be used to optimize the design of multi-orbit cone beam geometry to ensure complete sampling and to fully utilize the measured data as well. Second, the fully 3D algorithm previous developed by us will be extended to multi-orbit cone beam reconstruction. Third, a truly 3D algorithm will be derived for multifocal cone beam reconstruction. Finally, compensations for the degrading factors will be performed. The Chang algorithm will be extended to converging beam SPECT. To compensate for detector response, a preprocessing filtering will be derived based on the frequency-distance principle. Scatter compensation will be made using the newly developed slab technique and dual-photopeak window technique. To evaluate the proposed algorithms and techniques, a cone beam and a multifocal cone beam collimators will be designed and constructed. Experiments using the converging-hole collimators and Monte Carlo simulations will be carried out, and then the reconstructed images will be compared to the phantoms by means of various quantitation methods.